A polyoxometalate is composed of basic units consisting of MO4 tetrahedra, MO5 square pyramids, MO6 hexahedra or MO5 trigons in which oxygen atoms are 4-, 5- or 6-coordinated to metal atoms and the like. The basic units can be crosslinked by the oxygen atoms through dehydration condensation reaction and linked via vertices, edges or faces, so as to form polyoxometalates having various structures. Thus, recently, research and development of the polyoxometalate has been actively conducted since it can be utilized for various applications (Patent Document 1 and the like).